Zinc particles are widely used in different types of coating compositions. Such particles exist in three forms: powder, dust and flake. The primary differences between the powder and dust on the one hand and flake on the other lie in their aspect ratio and their density. The aspect ratio of zinc flake (that is the ratio of diameter to thickness) is typically in the range of about 5:1 to 40:1, and preferably a range of about 15:1 to 25:1, more preferably about 20:1. Such flakes frequently have a thickness of from 0.5 to 2 microns, for example about 1 micron. Zinc powder on the other hand tends to spheroidal shaped particles having a particle size in the range 15 to 40 microns, whereas dust is also formed primarily of spheroidal particles of a size of from 3 to 15 microns. Zinc flake refers to particles having a particle size of 1 to 100 micros, preferably 6–50 microns, frequently in the range 10 to 15 microns as measured by a Coulter Particle Size Analyzer. Zinc dust has a tapped density above 3. Whereas zinc flake has a tapped density below 3, for example in the range 2 to 2.5, commonly about 2.4.
Because of its greater covering power and lower density than zinc dust, zinc flake has always appeared to be an attractive material to use.
U.S. Pat. No. 5,478,878 (Nagaoka, et al.) mentions the possible use of zinc flake in a polyphenylene ether/polyamide composition having improved resistance to discoloration upon exposure to light.
Zinc flake, has been suggested for use in a number of anti-corrosion compositions. For example U.S. Pat. No. 5,338,348 (“the '348 patent”) discloses a coating composition for use in protecting metallic substrates from corrosion, comprising in weight percent, based on the total weight of the composition: from about 7% to 35% of film-forming substance; from about 35% to 55% of zinc powder (as defined above); from about 5% to 25% of zinc flakes; from about 1% to 5% at least one kind of amorphous silica; and up to about 30% particulate ferrophosphate.
U.S. Pat. No. 5,334,631 (Durand) describes a resin-based coating composition containing a mixture of zinc powder and zinc flake, it being taught that mineral spirits should be used with the flake to achieve a satisfactory composition.
Canadian Patent 2,074,329 relates to an improved powder coating composition comprising (a) a resin, (b) a curing agent and (c) zinc, wherein the zinc is a mixture of (c1) lamellar zinc (zinc flakes) and (c2) zinc dust.
A similar suggestion to use a combination of powder and flake is found in by Libuse Hochmannova in European Paint Journal, August 2002.
Despite the apparent desirability of using zinc flake rather than zinc dust, however, it has always been a problem to produce zinc flake at a price which is acceptable for broad-based use. For this reason, use of zinc flake has been largely confined to the coating of small parts in the fastener industry where cost is not a major consideration.
Typically metal flake particles are made by milling, for example ball milling, Such milling typically takes place in the presence of a lubricant. In principle two approaches are used either a wet method, in which stearic acid and mineral spirit are present or a dry method. The wet method is the principle method used for commercial production. However, the wet method is inherently expensive because flaking does not proceed rapidly, the materials of construction of the equipment used must be chosen to avoid contamination and the mineral spirits used are themselves flammable resulting in the need to take precautions to minimize the risk of fire. Furthermore a major problem in production of zinc flake has been removal of the solvent when a wet method is used since the presence of mineral spirits is not normally compatible with formulating the flake with conventional coating components such as epoxy resin or water-borne silicate-based compositions. Dry methods have, however, found only limited commercial acceptance.
U.S. Pat. No. 2,432,465 (Babcock) describes a method in which it is stated zinc or lead flake may be produced by a dry method. This is effected by disintegration of metal foil of a thickness of about 0.00065 inch in a stamp or hammer mill. Small pieces of aluminum foil may be used as lubricant, optionally in conjunction with an oily, greasy or fatty material.
U.S. Pat. No. 3,389,105 (Bolger) describes the production of metal flake from metal powder in a ball mil or stamping mill using a fluorocarbon resin as a grinding agent, optionally in combination with a material such as stearic acid. The milling technique may be wet or dry. The method is taught to be of particular use for production of “gold-bronze” flake but can be used for other metals.
U.S. Pat. No. 3,941,584 (Tindermann et al.) teaches ball milling of metal particles to produce flake. The teaching focuses on the production of aluminum flake but also describes production of zinc flake in a ball mill using a mixture of mineral spirits and stearic acid as a lubricant.
U.S. Pat. No. 4,318,747 (Ishijima) describes the production of flake pigments for use in a coting composition. Such flake particles are produced by, for example, wet ball milling using mineral spirits.
U.S. Pat. No. 4,469,282 (Booz) describes production of metal flake, particularly aluminum flake, by milling in the presence of a lubricant and a solvent. Long chain fatty acids such as stearic acid are suggested as possible lubricants and mineral spirits as solvent.
U.S. Pat. No. 4,820,552 (Espinosa et al.) teaches the production of metal flakes, such as zinc flakes by grinding metal particles in a hydrocarbon liquid in the presence of a surfactant and a metal oxide.
My U.S. Pat. No. 5,677,367 describes a soluble graphite containing zinc-rich composition so as to decouple manufacture of the dry composition from use of the solvent.